The invention relates to a method for inspecting an integrated circuit, the circuit comprising a plurality of sub-circuits and the method comprising a determination of a first supply current in a supply line of a first sub-circuit of the sub-circuits.
The invention further relates to an integrated circuit comprising a plurality of sub-circuits and comprising a current-measuring device for measuring a supply current in a supply line of at least one of the sub-circuits.
Such method and circuit are known from the Japanese Patent Application published under JP-A 62-278473. The known circuit comprises a number of sub-circuits and a current-detection circuit. Each sub-circuit has in its supply line a controllable switch which can connect the sub-circuit to the current-detection circuit in order to supply power to the sub-circuit. At a certain moment, one or more of the sub-circuits are connected to the current-detection circuit and the current-detection circuit measures the current to those connected sub-circuits. The not-connected sub-circuits do not receive power and are therefore not operational. The known circuit has a drawback in that the controllable switch in the supply line of the sub-circuit influences the behaviour of the sub-circuit even when such switch is closed. Depending on the implementation, the switch causes a voltage drop in the supply line and adversely affects the dynamic behaviour of the sub-circuit. In practice it is desirable to measure the current flowing into one sub-circuit at a time, because this will give the most accurate measurement of the current. The known circuit then has a disadvantage in that only the sub-circuit connected to the current-detection unit is operational and that the other sub-circuits remain in-operational. In the case that the sub-circuits are tightly coupled, as for example in a complex analogue integrated circuit, an in-operational sub-circuit may have an unpredictable influence on the current consumption in another, operational sub-circuit and may even damage a sub-circuit.
It is an object of the invention to provide a method of the kind set forth which influences the circuit to a lesser degree and which is better usable than the known method. This object is achieved according to the invention in a method that is characterised in that a first voltage over a segment of the supply line is measured, while all sub-circuits are operational, and that the determination of the first supply current is carried out on the basis of the first voltage and the resistance of the segment of the supply line. The method according to the invention has the advantage that no additional components are introduced in the supply line of the sub-circuit. The method uses the effect that a voltage drop occurs in the supply line itself due to the inherently present resistance of the supply line. After measuring the voltage drop over a given segment of the supply line, the supply current can be calculated based on the value of the resistance of the supply line. A further advantage of the method according to the invention is that a sub-circuits remains operational, even in the situation where its supply current is not determined. This means that possible interactions between sub-circuits all remain as in the normal operational mode and that no unpredictable currents will flow.
An embodiment of the method according to the invention, comprising a determination of a second supply current in a supply line of a second sub-circuit of the sub-circuits, is characterised in that a second voltage is measured over a segment of the supply line of the second-sub-circuit and that the first and the second voltage are measured by successively connecting a single voltage-measuring device to the segments of the first supply line and the second supply line respectively. A single voltage-measuring device now suffices for measuring the voltages over the various segments of the supply lines of the sub-circuits, instead of a voltage-measuring device for each individual segment.
An embodiment of the method according to the invention is characterised in that at least one of the voltages is measured by a differential pair of transistors. The topology of circuits in practice and the process for producing them are such that a differential pair of transistors can be employed for a range of voltage values to be measured. The differential pair of transistors is a very simple device of only two transistors and thus requires very few additional components to the circuit. Furthermore the input impedance of the differential pair is very high compared to the sub-circuit fed by the supply line and there will be no noticeable influence on the supply line by the differential pair.
An embodiment of the method according to the invention is characterised in that the at least one of the voltages is measured in the following steps:
connecting a first input of the differential pair of transistors to a first side of the particular segment, connecting a second input of the differential pair of transistors to a second side of the particular segment and making a first measurement,
connecting the first input of the differential pair of transistors to the second side of the particular segment, connecting the second input of the differential pair of transistors to the first side of the particular segment and making a second measurement,
combining the first measurement and the second measurement to a result for use as the measured voltage over the particular segment.
In this embodiment, the accuracy of a DC-measurement is increased by making the two measurements with interchanged connections to the particular segment, thereby eliminating the offset of the differential pair of transistors. This offset is present in both measurements and can be eliminated by combining the two measurements.
An embodiment of the method according to the invention is characterised in that the supply current is compared with a for that particular supply current determined range and that the circuit is rejected if the supply current falls outside the range. By comparing the measured supply current with a standard, to be expected value, faults in the circuit can be detected and if present the circuit can be rejected. Faults in a circuit may lead to a deviation of the supply current from the supply current in a completely correct circuit. By accurately measuring the supply current, the existence of such a fault in the circuit can be determined. The accuracy of such a test can be increased by subdividing the circuit into a number of sub-circuits each having fewer components than the original circuit. Then the deviation due to the fault can be better distinguished compared with the situation of a large circuit with many components. This method according to the invention can be used for testing the correctness of integrated circuits in various stages of the production process, e.g. as a test on a wafer with integrated circuits, but also for various tests on the finished product, e.g. a reliability test during the operational life of an integrated circuit.
An embodiment of the method according to the invention, comprising the determination of at least two supply currents for respective sub-circuits, is characterised in that a current ratio is determined between a first one and a second one of the two supply currents and that the integrated circuit is rejected if the current ratio falls outside a for the particular current ratio determined range. The production process of the integrated circuit will influence both supply currents in a similar way, e.g. a certain increase of the current. By comparing the ratio between the two currents of the same circuit with some standard value rather than comparing the absolute value of the current with a standard value, the influence of the production process on a test of the circuit is reduced.
It is a further object of the invention to provide a method for inspecting an integrated circuit, the method comprising a determination of a current in a signal line in the circuit, characterised in that a voltage over a segment of the signal line is measured and that the determination of the current is carried out on the basis of the voltage and the resistance of the segment of the signal line. The method according to the invention as described above for determining the supply current in one or more supply lines exploits the fact that a supply line inherently has a parasitic resistance. Now a signal line in a circuit also has an inherently present parasitic resistance. Therefore a current in such a signal line can be determined in a similar manner as the current in the supply line. In the method according to the invention, the signal current is determined in a non-invasive way, without the addition of components or the like in the signal line that might influence the behaviour of the circuit. The voltage over the segment of the signal line can be measured without causing noticeable influence on the circuit. These advantages are particularly important for signal lines that are sensitive to external disturbances.
It is a further object of the invention to provide an integrated circuit of the kind set forth in which a supply current in a sub-circuit can be measured in an improved way compared with the known integrated circuit. This object is achieved according to the invention in an integrated circuit that is characterised in that the current-measuring device comprises a voltage-measuring device for measuring a voltage over a segment of the supply line. In this circuit, the supply current in a supply line can be measured without any additional components in the supply line. Such additional components could influence the operational behaviour of the sub-circuit. The measurement of the voltage drop over a segment of the supply line and the resistance of this segment make it possible to determine the current through the supply line. Furthermore, the integrated circuit according to the invention has the advantage that a sub-circuit remains operational, also during the period where no measurement of the current for the particular sub-circuit is made.
An embodiment of the integrated circuit according to the invention is characterised in that the current-measuring device comprises connection means for successively connecting the voltage-measuring device to the segment of the supply line and to a segment of a supply line of a further one of the sub-circuits. The advantage of such a circuit is that a single voltage-measuring device suffices to make measurements over various segments of supply lines of various sub-circuits. This reduces the number of additional components necessary for the measurements.
An embodiment of the integrated circuit according to the invention, which circuit comprises a detection and/or diagnostic sub-circuit, is characterised in that the detection and/or diagnostic sub-circuit is arranged to process the result of the current-measuring device and to feed a result of the processing outside the circuit. By processing the result of the current-measuring device in the detection and/or diagnostic sub-circuit of the integrated circuit, an assessment of the circuit""s operation can be made internally, without the need of external equipment. Such an assessment can relate to an operational test to verify whether the circuit operates within specification limits. Furthermore, by regularly performing such an assessment the degradation of the circuit over time can be determined and the expected life time of the circuit can be estimated. Particularly in safety-critical applications such an assessment is valuable in order to determine the moment for replacement of the circuit. The result of the detection and/or diagnostic sub-circuit can be output in various ways, depending on the kind of assessment made. Examples are an output in the form of a simple pass/fail indication on a terminal of the circuit, and an output in the form of a signature of one or more words of several bits comprising a further qualification of the result. Such words can be serially output via a terminal of the circuit.
An embodiment of the circuit according to the invention is characterised in that the voltage-measuring device comprises a differential pair of transistors. The differential pair of transistors has a high impedance and therefore has a negligible influence on the supply line compared to the sub-circuit fed by the supply line. Because the differential pair requires two transistors, only few additional components are required to realise this voltage-measuring device.
It is a further object of the invention to provide an integrated circuit comprising a current-measuring device for measuring a current in a signal line in the circuit, characterised in that the current-measuring device comprises a voltage-measuring device for measuring a voltage over a segment of the signal line. Exploiting the fact the signal line has an inherently present resistance which causes a voltage drop over a segment of the signal line, is an advantageous method for determining the current in such a signal line. The method according to the invention is non-invasive and has a negligible influence on the behaviour of the circuit.